1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device with an electromagnetic (EM) type touch panel that has a thin profile and a simplified manufacturing process.
2. Discussion of the Related Art
For personal information processors, such as personal computers and mobile transmission devices, various input devices of keyboards, mice and digitizers have been generally used for text and graphic process. As personal information devices are in demand in various fields, the input devices of the keyboards and the mice have a limit in coping with the demand as an interface. Thus, it is beneficial to develop an input device that is easier to carry and simpler than those conventional input devices. That is, a user can input information such as letters on the input device with a bare hand while carrying the input device. Modem input devices have been developed not only to satisfy general input functions, but to have new functions and to provide them with great reliability and endurance using a micro-fabrication technology.
Especially, touch panels have attracted attention as an input device for their simplicity and mobility, on which a user easily inputs letters without additional input means. Thus, the detection method, function and structure of such touch panels are well known. Capable of sensing when a user touches a display surface, touch panels are classified into resistive type, capacitive type, and electromagnetic (EM) type. For the resistive type touch panels, two resistive sheets are stacked together with spacers at a predetermined interval. The two sheets contact each other, when an input means touches the surface of the touch panels. In operation, the location of a touched point is detected by reading a current change at the touched point in an applied D.C. voltage. Meanwhile, the capacitive type touch panels read a touched point by detecting a capacitance coupling in an applied A.C. voltage. The electromagnetic EM type touch panels detect a touched point using a resonant frequency of an induced voltage in an applied electromagnetic field. Each type has different characteristics of signal amplification, resolution, and difficulty of design and fabrication, so that a type is chosen for merit, such as optical, electrical, mechanical, resistance to ambient atmosphere and input characteristics as well as endurance and economical efficiency.
In recent years, the EM type touch panels have attracted great attention, because it is possible to detect the location of a touched point with precision. Hereinafter, a structure and an operation of a general EM type touch panel will be described.
The general EM type touch panel includes a flat substrate of a digitizer having two sets of array coils (or coils), with one set being arranged in perpendicular to the other, and a stylus pen for inputting data on the flat substrate of the digitizer. The coils of the flat substrate of the digitizer are comprised of a plurality of coils piled on a flexible Printed Circuit Board PCB, and each coil is arranged at a predetermined interval from X or Y axis. Also, one end of each coil is connected to a grounding voltage, and the other end is connected to a common reference potential line to which a select signal is applied.
In this system, the stylus pen includes a resonant circuit, and the flat substrate of the digitizer operates as it receives an A.C. signal from the array coils. When the stylus pen is positioned adjacent to the flat substrate of the digitizer, the array coils to which the A.C. signal is applied generate a magnetic field. Then, the resonant circuit of the stylus pen generates a resonant frequency in response to the magnetic field. Subsequently, the resonant frequency is detected in a controller of the touch panel, and the two-dimensional location of the touched point on the flat substrate is determined through a comparison process.
In the digitizer including the plurality of coils, the coils are formed of a conductive material having light-shielding characteristics. Thus, the coils are positioned below a light source of a display device to prevent a light leakage which may be generated by the coils. Unlike the resistive or capacitive type touch panels, which require the sensor unit to be positioned above the display devices, a sensor unit of the EM type touch panel may be positioned apart from a display surface, on which a touching process is performed. This is because the EM type touch panel detects a location of a touched point using an electromagnetic wave passing through the display device and the light source.
Hereinafter, an Electro-magnetic EM type touch panel according to the related art will be described with reference to the accompanying drawings.
FIG. 1 is a schematic view illustrating an LCD device including the EM type touch panel according to the related art. As shown in FIG. 1, the LCD device including the EM type touch panel according to the related art includes an LCD panel 10, an upper polarizing plate 11, a lower polarizing plate 12, a backlight unit 13, a sensor unit 14, a control unit 15, a metal case top (not shown) and an electronic stylus pen 17. The LCD panel 10 includes upper and lower substrates bonded to each other at a fixed interval and a liquid crystal injected between the upper and lower substrates for displaying picture images according to external driving and video signals. Also, the upper polarizing plate 11 is formed on the LCD panel 10, and the lower polarizing plate 12 is formed underneath the LCD panel 10, thereby polarizing light. The backlight 13 uniformly irradiates light at the rear of the LCD panel 10. Then, the sensor unit 14 is positioned below the LCD panel 10 for transmitting and receiving a resonant frequency at a touched point of the electronic stylus pen 17, thereby detecting the location of the touched point. The control unit 15 controls the sensor unit 14. Also, the metal case top (not shown) supports the LCD panel 10, the backlight 13, the sensor unit 14 and the control unit 15 as one body, and the electronic stylus pen 17 transmits an electromagnetic wave to the sensor unit 14, and receives the electromagnetic wave from the sensor unit 14.
The sensor unit 14 includes a sensor PCB including an X-axis coil array and a Y-axis coil array, a shield plate for preventing an external electromagnetic wave at a lower portion of the sensor PCB, and a connector including a switching means for selecting specific X-axis and Y-axis coils according to a transmitting mode or a receiving mode of the sensor PCB. In the control unit 15 positioned above the sensor unit 14 for transmitting a signal to the sensor unit 14 and reading an input signal from the sensor unit 14, a Control Processor unit (CPU) is positioned for detecting the location of a touched point by the electronic stylus pen 17. Also, the electronic stylus pen 17 includes a resonant circuit having a coil and a condenser therein.
The operation of the related art EM type touch panel will be described as follows.
First, the control unit 15 is operated by receiving the signal, so that the sensor unit 14 generates the electromagnetic wave by selecting the X-axis and Y-axis coils and by inducing an electromagnetic field. Thus, the resonant circuit of the electronic stylus pen 16 generates a resonant frequency in response to the electromagnetic wave, which is held for a predetermined time period. Then, the sensor unit 14 receives the signal, and detects the location of the touched point. At this time, the electronic stylus pen 17 includes the resonant circuit, which is a RLC complex circuit. The resonant circuit causes a maximum current to flow at a predetermined frequency of an applied power source. The resonance frequency obtains output characteristics of a predetermined frequency band. The resonance frequency (f) can be expressed by the following equation,f=½π√{square root over (LC)} (L is a coil inductance, and C is a condenser capacity).
The related art EM type touch panel is different from the resistive type touch panel in that the touched point by the electronic stylus pen is detected by using the resonance of the electromagnetic field, thereby detecting the correct location of the touched point. Further, the related art EM type touch panel endures and transmits well without deterioration on a picture quality. Also, in the related art EM type touch panel, it is possible to write letters on the touch panel with the electronic stylus pen without any effect from contact by a hand, so that the related art EM type touch panel is generally used in various fields for designs, conferences and seminars.
FIG. 2 is a block diagram illustrating a driving circuit and a driving method of the related art EM type touch panel. Referring to FIG. 2, the sensor unit 14 includes X-MUX and Y-MUX coupled to the X-axis and Y-axis, respectively. A specified Y-axis coil is selected by a Y address signal (Y-ADDR), and a specified X-axis coil is selected by an X address signal (X-ADDR) for reading. Both X and Y address signals are generated from the control unit 15.
After that, output signals from the selected Y-axis coil and the X-axis coil are provided to the control unit 15. The control unit 15 includes an amplifier 24 to grade and amplify the output signals. The output of the amplifier 24 is provided to an analog-digital converter 28 through a detector 25, a low-pass filter LPF 26, and a sample and hold S/H unit 27. The analog-digital converter 28 converts the size and polarity of an analog signal to a digital format, and then inputs the converted digital format to the CPU 23. Subsequently, the output of the amplifier 24 is provided to the detector 25, and then is provided to the low-pass filter 26 and the sample and hold S/H unit 27. While the analog-digital converter 28 is digitizing the signal, the sample and hold S/H unit 27 holds a measured value of one coil, and a next coil measurement starts at the front circuit part.
The sensor unit 14 includes a plurality of coils being piled up on a flat surface of a flexible PCB. Each coil is arrayed against X-axis and Y-axis, and has one side being connected to a ground voltage, and the other side being connected to one MUX unit in which one coil is selected for being connected to an electric potential line of a fixed level.
Upon use of the electronic stylus pen 17, a sine wave current 22 generated from a sine wave generator 21 is applied to the electronic stylus pen 17 under control of the CPU 23, whereby a sine wave magnetic flux is generated around the electronic stylus pen 17. As the electronic stylus pen 17 becomes adjacent to the sensor unit 14 by a user, each sine wave voltage in different sizes is induced to each coil arrayed in the sensor unit 14 and input to the CPU 23 through the detector 25 and the analog-digital converter 28. After that, the CPU 23 calculates the value of the position of the electronic stylus pen 17 on the sensor unit 14 from the value induced to the coil, and outputs the angle value between 0° and 360°. The output data of the electronic stylus pen 17 is applied to the LCD panel 10 or stored in the CPU 23.
It is more convenient for a user to draw a figure when an area of the sensor unit 14 of the EM type touch panel is larger, and is more efficient when the resolution is higher. The resolution is in inverse proportion to spaces between coils in the sensor unit 14. That is, when the spaces between the coils are narrower, the resolution becomes higher.
In the EM type touch panel, the plurality of coils are provided inside the sensor unit 14, so that it is possible to detect the touched point of the electronic stylus pen 17 by detecting electromagnetic changes. Accordingly, unlike the resistive type touch panel, it is not required to mount the sensor unit 14 at the front of the LCD panel 10 in the EM type touch panel. That is, the digitizer may be mounted at the rear of an Liquid Crystal Module (LCM). In case of the EM type touch panel transmitting electromagnetic force and having uniform electromagnetic substance, it is possible to detect the location of the electronic stylus pen moving on the LCD panel 10 by the sensor unit 14 even if the LCD panel 10 is positioned on the sensor unit 14.
However, the related art EM type touch panel has the following disadvantages.
In the sensor unit of the related art EM type touch panel, the coils are formed of a magnetic material. Thus, in case the sensor unit is positioned on the backlight, the light from the backlight is not transmitted due to the coils, thereby decreasing transmittance. In this respect, the sensor unit is positioned below the backlight in the related art EM type touch panel. Accordingly, when assembling the sensor unit to the backlight unit of the LCD panel and the driving circuit, a metal mold of 1 mm is mounted, in addition to the LCD panel, whereby the LCD device including the EM type touch panel becomes thick and heavy.
Also, the PCB having the driving circuit of the LCD panel is positioned below the backlight, so that it is required to form the shield plate below the PCB to protect the driving circuit from the sensor unit.